How to use default_value with customized data struct?

[GNUqb114514]

Hello,
I'm trying to build an app and I want to create a parameter in a customized struct
and give it a default value and I'm got stuck.
This is a small example:

use clap::Parser;

#[derive(Clone)]
pub struct Color {
    r: u8,
    g: u8,
    b: u8,
    a: u8,
}

impl Color {
    pub fn new(r: u8, g: u8, b: u8, a: u8) -> Self {
        Self { r, g, b, a }
    }
}

impl Into<[u8; 4]> for &Color {
    /// Translate this to byte reprensation,
    /// in ARGB8888 format.
    fn into(self) -> [u8; 4] {
        [self.a, self.r, self.g, self.b]
    }
}

impl From<Color> for clap::builder::OsStr {
    fn from(value: Color) -> Self {
        format!("#{:x}{:x}{:x}{:x}", value.r, value.g, value.b, value.a).into()
    }
}

impl core::str::FromStr for Color {
    type Err = String;

    fn from_str(s: &str) -> Result<Self, Self::Err> {
        // Some conditions
        Ok(Color::new(0,0,0,0))
    }
}

#[derive(Parser)]
pub struct Config {
    #[arg(value_parser=|v:&str| v.parse::<Color>(), default_value=Color::new(255, 0, 0, 255))]
    background_color: Color,
}

impl Config {
    pub fn background_color(&self) -> &Color {
        &self.background_color
    }
}
fn main() {
    println!("Hello, world!");
}

(some details are hidden for not distinguishing)
And I'm got:

$ cargo run
error[E0277]: the trait bound `clap::builder::OsStr: From<std::string::String>` is not satisfied
  --> src/main.rs:27:9
   |
27 |         format!("#{:x}{:x}{:x}{:x}", value.r, value.g, value.b, value.a).into()
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ ---- required by a bound introduced by this call
   |         |
   |         the trait `From<std::string::String>` is not implemented for `clap::builder::OsStr`, which is required by `std::string::String: Into<_>`
   |
   = help: the following other types implement trait `From<T>`:
             `clap::builder::OsStr` implements `From<&&'static std::ffi::OsStr>`
             `clap::builder::OsStr` implements `From<&&'static str>`
             `clap::builder::OsStr` implements `From<&'static std::ffi::OsStr>`
             `clap::builder::OsStr` implements `From<&'static str>`
             `clap::builder::OsStr` implements `From<&Str>`
             `clap::builder::OsStr` implements `From<&clap::builder::OsStr>`
             `clap::builder::OsStr` implements `From<Color>`
             `clap::builder::OsStr` implements `From<Str>`
   = note: required for `std::string::String` to implement `Into<clap::builder::OsStr>`

I'm temporary using String.leak() to make compiler happy,
but I want a better solution.

[epage]

impl From<Color> for clap::builder::OsStr {
    fn from(value: Color) -> Self {
        format!("#{:x}{:x}{:x}{:x}", value.r, value.g, value.b, value.a).into()
    }
}

This requires impl From<String> for OsStr. If you check the documentation, this requires the string feature, so run cargo add clap -F string. With clap, you have to opt-in to owned type support as it has a cost that most users don't need to pay.

The compiler would report this if you use something behind a cfg but it doesn't integrate it into name lookup so it doesn't help in cases like this.

[epage]

btw you probably want default_value_t rather than default_value. default_value_t is meant for default values in your native type while default_value is for OsStr defaults, representing the form used on the command line.

[GNUqb114514]

yes, you'd probably right, thanks.

…

________________________________ From: Ed Page ***@***.***> Sent: Monday, October 21, 2024 9:53:56 AM To: clap-rs/clap ***@***.***> Cc: GNUqb114514 ***@***.***>; Author ***@***.***> Subject: Re: [clap-rs/clap] How to use default_value with customized data struct? (Discussion #5783) btw you probably want default_value_t rather than default_value. default_value_t is meant for default values in your native type while default_value is for OsStr defaults, representing the form used on the command line. ― Reply to this email directly, view it on GitHub<#5783 (comment)>, or unsubscribe<https://github.com/notifications/unsubscribe-auth/A2KCXSGZDRXZFYU47GBD353Z4RNDJAVCNFSM6AAAAABQIMZFLSVHI2DSMVQWIX3LMV43URDJONRXK43TNFXW4Q3PNVWWK3TUHMYTAOJZHE4DAMI>. You are receiving this because you authored the thread.Message ID: ***@***.***>